This invention relates to a process for producing acicular magnetite or acicular maghemite of favorable particle properties, especially high long axis-to-short axis ratio and magnetic characteristics suitable for use in magnetic recording medium.
In recent years, a demand for a high quality of magnetic recording media including magnetic tape, magnetic disc and the like has been more and more increasing with the progress of miniaturization and weight saving of reproducing apparatus for magnetic recording. To be specific, magnetic recording media of higher bit density, higher output, higher sensitivity and improved frequency characteristic et al, has been requested.
The magnetic material should have a high coercive force Hc and a large saturation magnetization .sigma..sub.s suitable for satisfying the requests for the magnetic recording media. Further, these magnetic properties of magnetic recording media are mainly influenced by their residual magnetic flux density Br, while the residual magnetic flux density Br is strongly influenced by properties of the magnetic powder such as their dispersiveness in vehicle, their orientation and packing property in a coating medium.
Iron oxide needles such as acicular magnetite powder and acicular maghemite powder used as a main magnetic recording material is usually prepared in such a manner that acicular .alpha.-FeO (OH) is reduced in a stream of reducing gas such as H.sub.2, at a temperature of 300.degree. to 400.degree. C. to produce acicular magnetite powder, or thereafter the produced acicular magnetite powder is re-oxidized at a temperature of 200.degree. to 300.degree. C. in the air to produce acicular maghemite powder. The iron oxide needles obtained in this way have good magnetic properties, that is, relatively high coercive force Hc and relatively high rectangular Br/Bm ratio, nevertheless these magnetic properties are still unsatisfactory. Therefore many efforts have been made to improve the magnetic properties of acicular iron oxide particles.
It is known that such magnetic properties correlate largely to the shape and size of the starting acicular .alpha.-FeO(OH) particles. The shape and size, especially long axis-to-short axis ratio (hereinafter referred to as simply "axial ratio") of the particles which determine the magnetic properties including coercive force (Hc), saturation magnetization (.sigma..sub.s) et al. must be adjusted in the .alpha.-FeO(OH) itself.
The starting acicular .alpha.-FeO(OH) is prepared by, as a typical manner, adding more than equivalent of alkaline aqueous solution to an aqueous solution of ferrous salt to produce an aqueous suspension containing precipitated Fe(OH).sub.2, and thereafter oxidizing the obtained suspension at a pH of more than 11 and at a temperature of less than 80.degree. C. The acicular .alpha.-FeO(OH) thus obtained is in a form of needles having a length of long axis of about 0.5-1.5 .mu.m while the axial ratio is even up to about 10:1, and further their particles can not be said to have sufficient axial ratio. Moreover, the conversion of .alpha.-FeO(OH) into magnetic iron oxide has a considerable influence in changing the axial ratio and so the axial ratio of final product gets to be at most about 6:1.
One method for producing .alpha.-FeO(OH) particles with higher axial ratio is the addition of certain ion of foreign metal other than Fe into an aqueous solution of ferrous salt. The foreign metal is, for example, Co, Ni, Cr, Mn, Cd and facilitates to grow the particles in the direction of their long axis. However, this addition leads to the ultramicronization of particles. Unfortunately the ultramicronization is more conspicuous with the increase of the added amount of foreign metal. The thus-ultramicronized .alpha.-FeO(OH) particles are not suitable as a starting material for producing magnetic powder.
The another method is the addition of magnesium sulfate in an amount of 0.2 to 2% by weight based on the amount of the ferrous salt to the ferrous salt suspension at a pH of 7 to 11 in the presence of alkali carbonate. However, this method aims to obtain the .alpha.-FeO(OH) particles in a spindle-like shape, and in fact the particles obtained by this method have a spindle-like shape with a minute diameter. Besides this method also leads to the micronization of the particles. Therefore the ferromagnetic iron oxide particles produced from the thus-obtained .alpha.-FeO(OH) particles cannot be said to have sufficient magnetic properties.
Accordingly, an object of this invention is to provide a process for preparing acicular magnetite or acicular maghemite particles having high axial ratio.
Another object of this invention is to provide a process for preparing acicular magnetite particles or acicular maghemite without ultramicronization and suitable for use in magnetic recording medium.
These and other objects of this invention will become more apparent from the following description.
The foregoing and other objects of this invention will be accomplished by the process of this invention in which the formation of acicular .alpha.-FeO(OH) having high axial ratio as starting material is carried out by previously providing an iron (II) hydroxide (Fe(OH).sub.2) suspension at a pH of at least 11 with a magnesium sulfate or magnesium chloride and thereafter oxidizing the iron (II) hydroxide suspension with an oxygen-containing gas.